I got one suggestion: Don't remove those accounts from Shadowbot. Just freeze them (so they won't upvote and won't receive upvotes from the pool) until they are above the minimum.

The account should automatically defreeze with reaching the 5/10/15 SP, starting to give upvotes and receiving upvotes.

The reason is simple: Every account is useful sooner or later, they can set it up and forget about it.
A redfish/plankton etc. may find Shadowbot, likes it, wants to join but is not able to. At least let him join but don't use the account until it's worth it.
The user will forget about Shadowbot later on and with an account already set up and defreezing at the right time, it's a fully automated reminder.

The sign up and the regularly check on the SP should be quite resource friendly with a good return from those sign ups getting later above the minimum. Every user should be kept.

Also something else to consider: The reduced vote value by 1.2 SP.
https://steemit.com/utopian-io/@crokkon/how-will-the-hf20-dust-vote-changes-affect-the-vote-values

If a vote was worth 1.2 SP before, it will be worthless with HF20. A 15 SP vote will lose around 8% of its value in rshares. A 100 SP vote will give up 1.2%, a 1000 SP vote loses 0.12%. The more SP an account uses to vote, the less affected is its vote value.

So even with enough RC a 1.2 SP vote at 100% is still totally worthless. So regarding to that HF20: Improvise. Adapt. Overcome.